Camshaft phasers for varying the timing of valves in internal combustion engines are well known. A typical phaser comprises a rotor attached to a camshaft and a stator driven in time by a sprocket or pulley connected to an engine crankshaft. The phaser is able to vary the rotary position of the rotor with respect to the stator and thus to vary the valve timing imposed by the camshaft with respect to the crankshaft and pistons.
It is known to include a bias spring within a camshaft phaser, disposed functionally between the stator and the rotor, to urge the rotor in a predetermined direction to a predetermined position with respect to the stator when the rotor is not otherwise directed. See, for example, U.S. Pat. No. 6,276,321 B1. It can also be used to bias camshaft friction torque. Such a predetermined position may be, for example, either fully advanced or fully retarded. Typically, the desired predetermined position is fully retarded for an intake phaser and fully advanced for an exhaust phaser, wherein a locking pin may be activated between the rotor and stator to fix the timing at engine nominal for certain phases of engine operation.
In one example of the prior art, during manufacturing assembly of a phaser, the bias spring is wound by a spring retainer to its design-specified position and a retaining feature is installed to hold the spring in place. The rotor and the spring retainer have matching openings for receiving the retaining feature, for example, a screw or a pop rivet, in a predetermined unique angular relationship between the rotor and the spring retainer. In another example of the prior art, during manufacturing of the phaser, one end of the torsional bias spring is grounded to a fixed feature of the rotor, which may be a cup shaped member extending axially from the rotor, while the other end of the torsional bias spring is grounded to a fixed feature of the stator. The fixed feature of the stator may be a head of a bolt used to fasten the phaser end cover to the stator. The fixed features of the rotor/stator set a predetermined unique relationship between the ends of the torsional bias spring and thus the biasing spring force between the rotor and stator. The intention of such assembly processes is to have the spring wound to a desired and predetermined spring load. However, experience with prior art phasers assembled in such fashion shows that significant and unacceptable variation in spring loading can occur because of variation in spring rate within a population of springs provided for the purpose. Because the rotary action of the rotor requires overcoming the force of the bias spring, the speed of rotor response will vary inversely with the strength of the bias spring: the stronger the spring, the slower the response, especially when the oil control valve is energized. When a phaser is applied, for example, to one bank of a V-style engine, it is calculated that a variation in bias spring load of +/−10% can create a transient valve timing offset between the banks of up to 25%, which is clearly undesirable and unacceptable.
One solution to the problem is to reduce the variability range of spring constants in the population of springs. This solution requires a much higher level of quality control in spring manufacture, resulting in more expensive springs and thus more expensive phasers.
What is needed in the art is a means for installing and retaining any of the springs in the population of springs to a predetermined spring load.
It is a principal object of the present invention to provide a plurality of camshaft phasers having identical loading of their bias springs.